"Dear John" is an English idiom that refers to a letter, email, or message used to break up a romantic relationship.
Here are some ways you could convey a similar sentiment in Yoruba:
Formal:
* "Mo fẹ́ gbé e lẹ́nu iṣẹ́ pé..." (I want to inform you that...) followed by the reason for the break up.
* "Ó ti di pé..." (It has become clear that...) followed by the reason for the break up.
Informal:
* "Mo nifẹ́ láti sọ̀ ọ́ pé..." (I want to tell you that...) followed by the reason for the break up.
* "Mi o le jẹ́ pé..." (I can't be...) followed by a reason for the break up.
It's important to remember that Yoruba is a very nuanced language, and the best way to express a difficult sentiment like this would depend on the specific context and the relationship between the people involved.
You might also consider using "Máa rìn l'ọ̀nà rẹ́" (Go your own way) as a way to express a separation, but this is more general and doesn't convey the specific meaning of a "Dear John" letter.